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Advanced Silicon Materials for Photovoltaic Applications

Sergio Pizzini (Editor)
ISBN: 978-0-470-66111-6
422 pages
August 2012
Advanced Silicon Materials for Photovoltaic Applications (0470661119) cover image
Today, the silicon feedstock for photovoltaic cells comes from processes which were originally developed for the microelectronic industry. It covers almost 90% of the photovoltaic market, with mass production volume at least one order of magnitude larger than those devoted to microelectronics.

However, it is hard to imagine that this kind of feedstock (extremely pure but heavily penalized by its high energy cost) could remain the only source of silicon for a photovoltaic market which is in continuous expansion, and which has a cumulative growth rate in excess of 30% in the last few years. Even though reports suggest that the silicon share will slowly decrease in the next twenty years, finding a way to manufacture a specific solar grade feedstock in large quantities, at a low cost while maintaining the quality needed, still remains a crucial issue. Thin film and quantum confinement-based silicon cells might be a complementary solution.

Advanced Silicon Materials for Photovoltaic Applications has been designed to describe the full potentialities of silicon as a multipurpose material and covers:

  • Physical, chemical and structural properties of silicon
  • Production routes including the promise of low cost feedstock for PV applications
  • Defect engineering and the role of impurities and defects
  • Characterization techniques, and advanced analytical techniques for metallic and non-metallic impurities
  • Thin film silicon and thin film solar cells
  • Innovative quantum effects, and 3rd generation solar cells

With contributions from internationally recognized authorities, this book gives a comprehensive analysis of the state-of-the-art of process technologies and material properties, essential for anyone interested in the application and development of photovoltaics.

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Preface xiii

List of Contributors xvii

1. Silicon Science and Technology as the Background of the Current and Future Knowledge Society 1Sergio S. Pizzini

1.1 Introduction 1

1.2 Silicon Birth from a Thermonuclear Nucleosynthetic Process 2

1.3 Silicon Key Properties 2

1.3.1 Chemical and Structural Properties 2

1.3.2 Point Defects 7

1.3.3 Radiation Damage and Radiation Hardness 7

1.4 Advanced Silicon Applications 9

1.4.1 Silicon Radiation Detectors 9

1.4.2 Photovoltaic Cells for Space Vehicles and Satellite Applications 11

1.4.3 Advanced Components Based on the Dislocation Luminescence in Silicon 12

1.4.4 Silicon Nanostructures 14

References 15

2. Processes 21
Bruno Ceccaroli and Sergio S. Pizzini

2.1 Introduction 21

2.2 Gas-Phase Processes 23

2.2.1 Preparation and Synthesis of Volatile Silicon Compounds 23

2.2.1.1 Production and Utilization of SiHCl3 24

2.2.1.2 Production and Utilization of SiCl4 25

2.2.1.3 Production of SiH2Cl2 (and other Chlorosilanes) 26

2.2.1.4 Production and Applications of SiH4 27

2.2.1.5 Production of SiF4 29

2.2.1.6 Other Silicon Compounds 30

2.2.2 Purification of Volatile Silicon Compounds 30

2.2.3 Decomposition of Volatile Precursors to Elemental Silicon 30

2.2.3.1 Metal Reduction 30

2.2.3.2 Hydrogen Reduction 31

2.2.3.3 Thermal Decomposition of Volatile Silicon Precursors 32

2.2.4 Most Common Reactors 33

2.2.5 Recovery of By-Products 38

2.2.5.1 By-Products in the Case of Thermal Decomposition or Hydrogen Reduction of TCS 38

2.2.5.2 By-Products in Case of Thermal Decomposition of Silane 39

2.2.5.3 By-Products in the Case of Metal Reduction of Silicon Precursors 40

2.3 Production of MG and UMG Silicon and Further Refining Up to Solar Grade by Chemical and Physical Processes 40

2.3.1 MG Silicon Production 42

2.3.2 Metallurgical Refining Processes 47

2.3.3 Metal–Metal Extraction Processes 52

2.3.4 Solid/Liquid Extraction Techniques 54

2.3.5 Final Purification by Directional Solidification 55

2.3.6 Solar-Grade Silicon Production from Pure Raw Materials or Via the Direct Route 58

2.4 Fluoride Processes 59

2.5 Silicon Production/Refining with High-Temperature Plasmochemical Processes 61

2.5.1 Silicon Production Via Plasma Processes 62

2.5.2 Silicon Refining Via Plasma Processes 63

2.6 Electrochemical Processes: Production of Silicon Without Carbon as Reductant 64

2.7 Conclusions 68

References 70

3. Role of Impurities in Solar Silicon 79
Gianluca Coletti, Daniel Macdonald and Deren Yang

3.1 Introduction 79

3.2 Sources and Refinements of Impurities 79

3.3 Segregation of Impurities During Silicon Growth 86

3.3.1 Equilibrium Segregation Coefficients 86

3.3.2 Effective Segregation Coefficient 87

3.3.3 Distribution of Impurities in Silicon Crystal Due to Segregation 90

3.4 Role of Metallic Impurities 92

3.4.1 Solubility and Diffusivity 92

3.4.2 Impact on Charge-Carrier Recombination 94

3.4.3 Modeling the Impact of Metallic Impurities on the Solar-Cell

Performance 96

3.5 Role of Dopants 101

3.5.1 Carrier Mobilities in Compensated Silicon 101

3.5.2 Recombination in Compensated Silicon 103

3.5.3 Dopant-Related Recombination Centers 105

3.5.4 Segregation Effects During Ingot Growth 106

3.5.5 Detecting Dopants in Compensated Silicon 107

3.6 Role of Light Elements 108

3.6.1 Oxygen 108

3.6.2 Carbon 109

3.6.3 Nitrogen 111

3.6.4 Germanium 113

3.7 Arriving at Solar-Grade Silicon Feedstock Definitions 114

References 118

4. Gettering Processes and the Role of Extended Defects 127
Michael Seibt and Vitaly Kveder

4.1 Introduction 127

4.2 Properties of Transition-Metal Impurities in Silicon 130

4.2.1 Solubility of Transition Metal Impurities 131

4.2.1.1 Solubility in Intrinsic Silicon 131

4.2.1.2 Solubility in Extrinsic Silicon 134

4.2.2 Diffusion of Transition-Metal Impurities 136

4.3 Gettering Mechanisms and their Modeling 139

4.3.1 Segregation Gettering 140

4.3.1.1 Segregation into a Second Phase 140

4.3.1.2 Segregation Gettering by Highly Doped n-Type Silicon 141

4.3.1.3 Segregation Gettering by Highly Doped p-Type Silicon 142

4.3.2 Relaxation Gettering 142

4.3.3 Injection Gettering 142

4.3.4 Modeling of Gettering Kinetics 143

4.3.5 Aluminum Gettering 144

4.3.6 Phosphorus-Diffusion Gettering 146

4.3.7 Boron-Diffusion Gettering 149

4.4 Bulk Processes Affecting Gettering Efficiency and Kinetics 150

4.4.1 Metal-Silicide Precipitates 150

4.4.1.1 Precipitate Composition and Misfit Strain 151

4.4.1.2 Electrial Actvity of Silicide Precipitates 152

4.4.1.3 Effect on Gettering Kinetics 152

4.4.1.4 Modeling of Precipitation Kinetics 153

4.4.2 Dislocations 154

4.4.2.1 Electronic Structure of Dislocations 155

4.4.2.2 Carrier Decombination at Dislocations 159

4.4.2.3 Interactions of Transition-Metal Impurities with Dislocations 162

4.4.2.4 Dislocation Luminescence: A Tool to Probe Gettering at Dislocations 165

4.4.3 Grain Boundaries 167

4.4.4 Light-Element Impurities and Related Defects 169

4.5 Gettering Strategies and Defect Engineering 170

4.6 Conclusions 173

References 174

5. Advanced Characterization Techniques 189
Anna Cavallini, Daniela Cavalcoli and Laura Polenta

5.1 Introduction 189

5.2 Surface Photovoltage Spectroscopy 190

5.2.1 The Basic Principles 191

5.2.2 SPS Setup 193

5.2.3 Surface Photovoltage Spectroscopy of Hydrogenated Nanocrystalline Silicon (nc-Si:H) 194

5.3 Photocurrent Spectroscopy 196

5.3.1 Basic Principles 197

5.3.2 Spectral Photoconductivity Setup 199

5.3.3 Application of Spectral Photoconductivity to Silicon and Silicon Devices 201

5.4 Optical (Light) Beam Induced Current (OBIC or LBIC) 202

5.4.1 Basic Principles of Optical Beam Induced Current Method 202

5.4.2 Determination of the Electric Field and Depletion Region Extent in Particle Detectors by OBIC 204

5.5 Scanning Probe Microscopy for the Nanoscale Electrical Characterization of Semiconductors for PV Applications 207

5.6 Concluding Remarks 210

References 210

6. Advanced Analytical Techniques for Solar-Grade Feedstock 215
Richard S. Hockett

6.1 Introduction 215

6.2 Review of Analytical Techniques 216

6.3 GDMS Analysis of PV Si 222

6.4 SIMS Analysis of PV Si 223

6.5 Applications of SIMS and GDMS for PV Si Feedstock Studies 227

6.5.1 Impurity Segregation in Directional Solidified (DS) Silicon Blocks 227

6.5.2 Specification of [C], [O] and [N] in Solar-Grade Silicon Feedstock to be Used in DS Furnaces 229

6.5.3 SIMS Capability for Reduced-Cost Measurement of [C, O, B, P] 230

6.5.4 Problems in Conversion Between Resistivity and Dopant

Concentration in Highly Compensated Silicon 231

References 232

7. Thin-Film Deposition Processes 235
J.K. Rath

7.1 Introduction 235

7.2 Deposition Techniques of Thin-Film Silicon 235

7.2.1 Standard Radio-Frequency Plasma-Enhanced CVD 236

7.2.2 Very High Frequency Plasma-Enhanced CVD 236

7.2.3 Microwave Plasma-Enhanced CVD 237

7.2.4 ETP 237

7.2.5 Low-Energy Plasma-Enhanced PECVD 238

7.2.6 Hot-Wire CVD 238

7.3 In Situ Diagnosis of Growth Conditions 239

7.3.1 Electrical: Current–Voltage (I –V ) Probe 239

7.3.2 Optical Emission Spectroscopy (OES) 240

7.3.3 Infrared Spectroscopy 243

7.3.4 Ellipsometry 244

7.3.5 Ion Energy Probe 245

7.4 Challenges of Deposition at High Growth Rates and Low Substrate Temperatures 246

7.4.1 Growth-Process Models 246

7.4.2 Inhomogeneity of Growth 250

7.4.3 Growth at High Deposition Rates 251

7.4.4 Silane Dissociation Efficiency and Depletion Criteria for nc-Si Deposition 252

7.4.5 Low-Temperature (LT) Deposition 254

7.4.6 Structural Evolution at Low Temperature 257

7.4.7 Transient Plasma 260

7.4.7.1 Turn-on Transient: Electrical 261

7.4.7.2 Seconds Range: Cluster Formation and Dusty Regime 261

7.4.7.3 Dependence of the Dust-Formation Regime (Deposition Parameter Variation) 264

7.4.7.4 Tens of Seconds Range: Gas-Phase Instability 266

7.4.7.5 Hundreds of Seconds Range: Long-Term Instability 270

7.5 Upscaling to Large-Area and Industrial Processing: Critical Analysis of Various Fabrication Processes 270

References 273

8. Modeling of Thin-Film Deposition Processes 287
Carlo Cavallotti

8.1 Introduction 287

8.2 Modeling the Plasma Discharge 290

8.3 Modeling of the Gas Phase and Surface Kinetics 295

8.3.1 Gas-Phase Kinetic Scheme 297

8.3.2 Surface Kinetic Scheme 301

8.3.3 On the Consistent Solution of the Plasma Discharge and Kinetic Models: Theory and Examples 303

8.4 Modeling of the Thin-Film Morphological Evolution 303

8.5 Status of the Field and Perspectives 308

References 309

9. Thin-Film Silicon Solar Cells 311
J.K. Rath

9.1 Introduction 311

9.2 Second-Generation Solar Cells: Advantages Compared to the First Generation 312

9.3 Drift-Type Thin-Film Silicon Solar Cells: Substrates and Configuration 314

9.4 Material Considerations for Thin-Film Silicon Solar Cells 316

9.4.1 Amorphous Silicon 316

9.4.2 Amorphous Silicon-Germanium 317

9.4.3 Nanocrystalline Silicon 317

9.4.4 Light Confinement 318

9.4.4.1 Front Contact 319

9.4.4.2 Back Contact 320

9.4.4.3 Antireflection Coating 321

9.4.4.4 Intermediate Reflector 321

9.5 Present Status of Drift-Type Thin-Film Silicon Solar Cells 321

9.5.1 Recent R&D Results on Thin-Film Silicon Solar Cells 322

9.5.2 Industrial Scenario 322

9.6 Technological Issues 325

9.6.1 High Deposition Rate 325

9.6.2 Thin Cells 325

9.6.2.1 Intermediate Reflector 327

9.6.2.2 Optically Thick–Electrically Thin 328

9.6.2.3 Plasmonic-Enhanced Backreflection (BR) Solar Cell 329

9.7 Third-Generation Thin-Film Silicon Cell 329

9.8 Solar Cells on Plastics 331

9.8.1 Transfer Method 331

9.8.2 Direct Deposition 332

9.9 Hybrid Cells 334

9.10 Industrial Scenario of Thin-Film Silicon-based Solar Cells 336

9.11 Challenges for Thin-Film Silicon Solar-Module Fabrication 338

References 341

10. Innovative Quantum Effects in Silicon for Photovoltaic Applications 355
Zhizhong Yuan, Aleksei Anopchenko and Lorenzo Pavesi

10.1 Basic Principles of 3rd-Generation Solar Cells 355

10.1.1 The Need for a New Generation of Solar Cells 355

10.1.2 Limitations in Early Generations 356

10.1.3 3rd-Generation Options 357

10.1.3.1 Tandem Solar Cells 357

10.1.3.2 Hot-Carrier Solar Cells 358

10.1.3.3 Solar-Spectrum Modification 358

10.1.3.4 Intermediate-Band Solar Cells 359

10.1.3.5 Multiple-Exciton Generation 359

10.2 The Advantages of Using Silicon Nanocrystals 359

10.2.1 Fabrication and Advantages of Si-NC 359

10.2.2 Quantum Confinement Effect in Si-NCs 360

10.3 Applications of Si-NC in the 3rd-Generation Solar Cells 362

10.3.1 All-Silicon Tandem Solar Cells 362

10.3.2 Hot-Carrier Solar Cells 364

10.3.3 Intermediate-Band Solar Cells 366

10.3.4 Multiple-Carrier Generation 368

10.3.5 Downshifter Cell 372

10.4 Challenges and Solutions 375

10.4.1 Size Control 375

10.4.2 Carrier Transport 376

10.4.3 Absorption 378

10.4.4 Technological Constraints 380

10.5 Conclusions 381

References 382

Index 393

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“It should be a good reference for people who are interested in silicon materials. Although many existing volumes are available in similar topics, this book should be the most up-to-date one for PV silicon.”  (Energy Technology, 1 October 2013)

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